Postactivation Potentiation: A Brief ReviewRoxanne Horwath and Len Kravitz, Ph.D.Introduction
Enhancing the acute and chronic effects of resistance training on a person's overall athletic performance is the goal of many researchers, strength and conditioning professionals and personal trainers. To that end, many resistance training methods, training strategies and ergogenic aids have been investigated. Some of the underlying mechanisms of these strategies include increased motor unit recruitment, increased muscle spindle firing, increased activity of the synergist musculature, reduced inhibition of the Golgi tendon organ and a phenomenon called postactivation potentiation (Hilfiker, Hubner, Lorenz & Marti, 2007). Postactivation potentiation (PAP) is a phenomenon that has recently gained popularity in the strength training community because it offers a proposed approach for optimizing force and power production above and beyond performance achieved without the use of PAP (Robbins, 2005). This phenomenon describes the enhanced and immediate muscle force output of explosive movements after a heavy resistance exercise is performed (Robbins, 2005). The PAP phenomenon can potentially maximize performance of explosive based activities such as weightlifting, sprinting, jumping and throwing activities (French, Kraemer, Cooke, 2003; Hilfiker, Hubner, Lorenz & Marti, 2007).

Two Theories of PAPThe underlying principle surrounding PAP is that prior heavy loading induces a high degree of central nervous system stimulation, resulting in greater motor unit recruitment and force, which can last from five-to-thirty minutes (Chiu, Fry, Weiss, et al., 2003; Rixon, Lamont, & Bemden, 2007). There are two proposed theories for PAP. The first theory involves an increased phosphorylation (addition of a phosphate for the production of ATP) of myosin regulatory light chains (proteins of muscle contraction) during a maximum voluntary contraction (MVC). This allows the actin (the other protein of muscle contraction) and myosin binding (for muscle contraction) to be more responsive to the calcium ions released (from the sarcoplasmic reticulum), triggering a cascade of events leading to enhanced force muscle production at the structural level of muscle (Hamada, Sale, & MacDougall 2000). The greater the muscle activation, the greater the duration of calcium ions in the muscle cell environment (referred to as sarcoplasm) and the greater the phosphorylation of the myosin light chain protein (Rixon, Lamont, Bemden, 2007). As a result, faster contraction rates and faster rates of tension develop (Chiu, Fry, Weiss, et al. 2003).

The second theory involves the Hoffmann Reflex (H-Reflex), named after the scientist (Paul Hoffmann) who first described it. The H-reflex is an excitation of a spinal reflex elicited by the Group Ia afferent muscle nerves (specialized nerves conducting impulses to muscle. It is theorized that the PAP intervention enhances the H-reflex, thus increasing the efficiency and rate of the nerve impulses to the muscle (Hodgson, Docherty, Robbins, 2005).

Muscle Fiber Type and PAPIt has been assumed that muscles with shorter twitch contraction time show predominance in fast twitch (Type II) muscle fibers and exhibit greater force than those muscles with longer twitch contraction times such as slow twitch (Type I) fibers. It was the purpose of a study conducted by Hamada et al. (2000a) to investigate the correlation between muscle fiber type distribution and PAP in human knee extensor muscles. The study was completed in two phases. The first phase tested a group of 20 male subjects. The subjects were measured by a dynamometer and were hooked up to an EMG machine that measured muscle twitch response of a 10-second MVC. In the second phase of the study, four subjects with the highest and lowest PAP scores underwent a needle biopsy of the vastus lateralis to determine the distribution of fiber type. The results showed that PAP is most effective when Type II fibers are at a greater percentage of the muscles being used. Thus, this phenomenon can be correlated to an increased performance in athletes and recreational enthusiasts who rely on a shorter twitch contraction time for optimal athletic performance in spurt activities such as sprinting, jumping and throwing.

Athletes vs. Recreationally Trained Individuals and PAPA study conducted by Chui and colleagues (2003) investigated the impact of training status on the response to PAP in athletes involved in explosive strength activities compared to individuals involved in recreational training. Twelve men and twelve women performed jump squats at five minutes and 18.5 minutes following a controlled (moderate intensity) or heavy (high intensity) PAP intervention over four sessions. The results of the study found that recreationally trained athletes exhibited fatigue at five minutes following the acute heavy resistance stimulus, and thus no enhanced performance. However, in the athletically trained individuals, the heavy PAP stimulus enhanced power performance at five and 18.5 minutes. The author's concluded that PAP enhances explosive strength performance in highly trained individuals, due largely to their fatigue-resistant, high level of conditioning.

PAP Effects on Endurance TrainingEndurance athletes typically have lower percentages of fast twitch (Type II) muscle fibers when compared to slow twitch (Type I) muscle fibers. Past research has shown a greater PAP response is seen in individuals engaging in activities that involve more Type II fiber types (Hamada, Sale, & MacDougall, 2000). However, researchers have revealed that endurance trained individuals also show an increase in the maximum shortening velocity of their Type I fibers after a PAP intervention (Hamada, Sale, MacDougall). Additionally, Hamada et al. believe that endurance athletes have an increased resistance to fatigue, allowing the PAP effect to prevail over fatigue. Hamada, Sale, and MacDougall conducted a study on triathletes, distance runners, an active control, and sedentary individuals. Each group contained 10 subjects which performed 10-second maximal isometric contractions of the elbow extensors and ankle planter flexors. Twitch responses were then elicited at five-seconds, one, three, and five minutes post-MVC. The results showed that triathletes who trained both the upper and lower body muscles had enhanced PAP response in both the elbow extensors and plantar flexors, when compared to sedentary individuals. The runners, who only trained lower body muscles, were found to have enhanced PAP reaction in the planterflexors but not in the elbow extensors. The active control group who trained both upper and lower body muscles had enhanced PAP effect in both muscle groups, but did not have as significant of an increase as the triathletes. The authors concluded that PAP can indeed enhance endurance athlete performance by offsetting fatigue. However, this enhancement is limited to the muscle groups that are trained, and is somewhat proportional to the training status of the individual.

Summary RemarksThe main goal of incorporating PAP is to increase force development (rate and quantity) to maximize explosive power for athletic performance. Research has shown that PAP does in fact exist and can enhance performance. There are a variety of differing strategies and methods for eliciting PAP, with no known approach being identified as the most preferred. However, the conclusion of the studies reviewed for this article point out a few concrete concepts. First, PAP is best for activities that require explosive power movements, such as sprinting, high jumping, ski jumping, weight lifting, and boxing (French, Kraemer, Cooke, 2003; Hilfiker, Hubner, Lorenz & Marti, 2007). Second, the PAP ergogenic stimulus has been found to last between two-to-thirty minutes (Chiu, Fry, Weiss, et al. 2003; Rixon, Lamont, Bemden, 2007). Lastly, the preconditioning load amount used in the PAP intervention is dependent on the type on contractile activity used in the physical activity, which needs further research elucidation (Hilfiker et al.). However, from this review it may also be concluded that each individual athlete is uniquely different, and what might work for one athlete, might not work for another.

Message to Personal TrainersA major purpose of this article was to introduce PAP to personal trainers and fitness professions and get a quick review of this phenomenon, its underlying physiology and its potential ergogenic effect. As scientists advance forward and develop most consistent strategies with PAP usage, surely methods of adaptation will be presented (as have been with periodization models) to also help recreationally trained clients optimally enhance their muscular fitness performance.

Side Bar 1. What is 'Complex' Training?The theory of 'complex' training incorporates a training stimulus that involves coupling heavy and light loads alternately in an orderly sequence to lead to a higher PAP response (French, Kraemer, Cooke, 2003). For example, a typical complex training exercise could pair a maximal contraction exercise, such as a squat, which is immediately followed by a plyometric exercise such as a depth jump (stepping off a box and then exploding upward upon ground contact) (French, Kraemer, Cooke, 2003). This training protocol offers an exercise sequence that enhances the involvement of the nervous system by heightening central nervous system excitability (French, Kraemer, Cooke, 2003). Although some research has been completed with 'complex training', much more research is needed in this reactive training strategy.

Bios:Roxanne Horwath, ATC, LAT, is a licensed and certified Athletic Trainer. She is a graduate student at the University of New Mexico currently majoring in Exercise Science. Roxanne is planning on pursing a degree and career as a Physician Assistant after obtaining her masters degree.

Len Kravitz, Ph.D., is the Program Coordinator of Exercise Science and Researcher at the University of New Mexico where he recently won the "Outstanding Teacher of the Year" award. Len was honored with the 1999 Canadian Fitness Professional International Presenter of the Year and the 2006 Canadian Fitness Professional Specialty Presenter of the Year awards and chosen as the American Council on Exercise 2006 "Fitness Educator of the Year.